/Engine/Source/Runtime/SlateCore/Public/Input/Reply.h
An event should return aFReply::Handled().CaptureMouse( SomeWidget ) as a means of asking the system to forward all mouse events to SomeWidget
[FReply](API\Runtime\SlateCore\Input\FReply) & CaptureMouse ( [TSharedRef](API\Runtime\Core\Templates\TSharedRef)< [SWidget](API\Runtime\SlateCore\Widgets\SWidget) > InMouseCaptor )